1. Field of the Invention
The present invention relates to (cyclo)aliphatic polyisocyanates which are partially or completely blocked with specific malonic esters, and to a process for their preparation.
2. Description of the Background
The blocking of aliphatic diisocyanates with malonic esters has been known for a long time. This reaction was first described in D.R.P. 756 058 (1942). The blocking of isophorone diisocyanate (IPDI) with malonic esters is disclosed in DE-A 24 36 872, for example, and the corresponding blocking of the trimeric IPDI in DE-A 30 01 060.
DE-A 24 56 469 describes a process for the preparation of water-soluble or water-dispersible polyisocyanates and their use as coating compositions, the blocked isocyanates being malonic ester-blocked biuretic of hexamethylene diisocyanate.
DE-A 25 50 156 describes capped polyisocyanate mixtures which are readily soluble in paint solvents and which can be formulated in combination with polyester polyols to give one-component PUR stoving enamels. These blocked polyisocyanates are reaction products of trimethylolpropane (1 OH equivalent) and IPDI (2-12 NCO equivalents), whose free NCO groups are blocked with malonic esters, specifically diethyl malonate.
The principal disadvantage of the known malonic ester-blocked polyisocyanates is their relatively high deblocking temperature. Hence, a need exists for malonic ester-blocked polyisocyanates having lower deblocking temperatures.